CIDR (Classless Inter Domain Routing)

CIDR stands for Classless Inter-Domain Routing. It is a way to define and manage IP address ranges more efficiently in computer networks. CIDR is commonly used in cloud networking, routing, and firewall rules to control which IP addresses are allowed to communicate.

What CIDR Means in Simple Terms

CIDR is a shorthand method for describing a group of IP addresses.

Instead of listing every IP address one by one, CIDR lets you define a whole range using a single expression.

For example:
192.168.1.0/24

This represents a block of 256 IP addresses, from 192.168.1.0 to 192.168.1.255.

How CIDR Notation Works

A CIDR block has two parts:

IP Address

This is the starting point of the range.

Example:
10.0.0.0

Prefix Length

The number after the slash, such as /16 or /24, defines how large the range is.

• /32 means one single IP address
• /24 means 256 IP addresses
• /16 means 65,536 IP addresses

The smaller the number after the slash, the larger the network.

Simple Examples

203.0.113.5/32
Represents one specific IP address

192.168.0.0/24
Commonly used for small private networks

10.0.0.0/8
Used for very large private networks

Where CIDR Is Used

Networking

Defines network ranges inside virtual private clouds and subnets.

Security Rules

Used in firewall rules to allow or block traffic from specific IP ranges.

Routing

Helps routers decide where to send traffic efficiently.

Cloud Infrastructure

Used heavily in AWS, Azure, and Google Cloud to design network layouts.

Why CIDR Is Important

Efficiency
Reduces wasted IP addresses

Scalability
Makes it easier to design large networks

Flexibility
Allows custom sized networks instead of fixed classes

Clarity
Provides a standard way to describe IP ranges

CIDR in Everyday Cloud Use

When you create a virtual network, you are often asked to choose a CIDR block. This defines how many systems your network can support and how traffic flows between services.

Choosing the right CIDR range early helps avoid future networking issues.

In Short

CIDR is a compact and flexible way to define IP address ranges. It is a fundamental concept in modern networking and cloud infrastructure, used to control traffic, design networks, and manage security.